Precision machine vision inspection systems (or “vision systems” for short) can be utilized to obtain precise dimensional measurements of inspected objects and to inspect various other object characteristics. Such systems may include a computer, a camera and optical system, and a precision stage that is movable in multiple directions so as to allow the camera to scan the features of a workpiece that is being inspected. One exemplary prior art system that is commercially available is the QUICK VISION® series of PC-based vision systems and QVPAK® software available from Mitutoyo America Corporation (MAC), located in Aurora, Ill. The features and operation of the QUICK VISION® series of vision systems and the QVPAK® software are generally described, for example, in the QVPAK 3D CNC Vision Measuring Machine User's Guide, published January 2003, and the QVPAK 3D CNC Vision Measuring Machine Operation Guide, published September 1996, each of which is hereby incorporated by reference in their entirety. This product, as exemplified by the QV-302 Pro model, for example, is able to use a microscope-type optical system to provide images of a workpiece at various magnifications, and move the stage as necessary to traverse the workpiece surface beyond the limits of any single video image. A single video image typically encompasses only a portion of the workpiece being observed or inspected, given the desired magnification, measurement resolution, and physical size limitations of such systems.
Image acquisition may be performed while using relative motion between the camera and the workpiece stage, thereby significantly increasing inspection throughput. It is advantageous for such systems to include strobe lighting illumination to assist with the acquisition of images during continuous motion without smearing (or blurring) the image. One exemplary method for acquiring images using continuous motion operations that can be used on different machine vision systems is described in U.S. Pat. No. 7,499,584, which is hereby incorporated by reference in its entirety.
General purpose precision machine vision inspection systems, such as the QUICK VISION™ system, are also generally programmable to provide automated video inspection. U.S. Pat. No. 6,542,180 (the '180 patent) teaches various aspects of such automated video inspection and is incorporated herein by reference in its entirety. As taught in the '180 patent, automated video inspection metrology instruments generally have a programming capability that allows an automatic inspection event sequence to be defined by the user for each particular workpiece configuration. This can be implemented by text-based programming, for example, or through a recording mode which progressively “learns” the inspection event sequence by storing a sequence of machine control instructions corresponding to a sequence of inspection operations performed by a user with the aid of a graphical user interface, or through a combination of both methods. Such a recording mode is often referred to as “learn mode” or “training mode” or “record mode.” Once the inspection event sequence is defined in “learn mode,” such a sequence can then be used to automatically acquire (and additionally analyze or inspect) images of a workpiece during “run mode.”
The machine control instructions including the specific inspection event sequence (i.e., how to acquire each image and how to analyze/inspect each acquired image) are generally stored as a “part program” or “workpiece program” that is specific to the particular workpiece configuration. For example, a part program defines how to acquire each image, such as how to position the camera relative to the workpiece, at what lighting level, at what magnification level, etc. Further, the part program defines how to analyze/inspect an acquired image, for example, by using one or more video tools such as edge/boundary detection video tools.
Video tools (or “tools” for short) and other graphical user interface features may be used manually to accomplish manual inspection and/or machine control operations (in “manual mode”). Their set-up parameters and operation can also be recorded during learn mode, in order to create automatic inspection programs, or “part programs”. Video tools may include, for example, edge/boundary detection tools, autofocus tools, shape or pattern matching tools, dimension measuring tools, and the like.
Part programs for acquiring images of edge features typically specify a level of magnification. When selecting a level of magnification, various tradeoffs may be considered. For example, higher levels of magnification may provide higher resolution, but also have a smaller field of view with respect to the overall workpiece and may result in greater distortion, in addition to requiring more expensive hardware for the magnifying elements. Lower levels of magnification may provide a larger field of view and less distortion, as well as lower cost, but may not provide the desired level of resolution and corresponding accuracy for certain applications. In some such cases, the resolution of an image of an object is limited by the pixel spacing in the camera detector, in that the spatial sampling on the object is determined by the pixel spacing and the magnification. A method is known to address this situation by acquiring a plurality of images that are stepped or offset by a known sub-pixel increment relative to one another, and the multiple sets of image data are then combined to effectively increase the image sampling density on the object. However, in some cases such methods have been too complicated to be understood and implemented by relatively unskilled users, or too slow to be practical in many industrial environments and/or applications. Some prior art systems have proposed a dithering motion of the camera or an optical component in order to provide the desired offset “automatically”. However, such methods are mechanically complicated, and may introduce vibrations and/or non-repeatability that are incompatible with precision inspection tolerances. An improved method and system that allows a desired level of resolution and accuracy to be obtained by relatively unskilled users, while utilizing relatively less expensive systems (e.g. existing systems) and lower levels of magnification would be desirable.